1. Field of the Invention
The present invention relates to improvements in an electric power steering apparatus carried on a vehicle.
2. Description of the Related Art
In recent years, electric power steering apparatuses have increasingly been used to provide a comfortable steering touch or feel to the driver by reducing the manual steering effort or force needed to turn the steering wheel. The electric power steering apparatuses of the type concerned are constructed such that an assist torque generated by an electric motor in proportion to the steering torque is transmitted to a steeling system. One example of such electric power steering apparatuses is disclosed in Japanese Patent Laid-open Publication No. HEI 7-165089, entitled "STEERING APPARATUS".
The disclosed electric power steering apparatus includes a rack shaft having a rack of gear teeth at its one end portion and a worm screw at its other end portion, a pinion meshing with the rack, and a ball-screw mechanism (often referred to as "ball nut mechanism") having a nut engaged threadedly with the screw and driven in rotation by an electric motor. A rack guide is disposed in confronting relation to the pinion with the rack shaft disposed therebetween so as to guide a longitudinal reciprocating movement of the rack shaft. The rack guide is urged by a spring toward the rack shaft so that backlash between the rack and the pinion is removed.
The conventional electric power steering apparatus of the foregoing construction has a drawback, as will be understood from a description given below with reference to FIGS. 10A, 10B, 10C and 10D.
FIG. 10A diagrammatically shows in plan view a vehicle steering system in which the electric power steering apparatus 100 is incorporated. In this figure, the rack shaft, the ball-screw mechanism, the rack, the pinion and the spring-loaded rack guide are designated by 101, 105, 106, 107 and 108, respectively. Numerals 102, 102 are tie rods connected at one end to the opposite ends of the rack shaft 101, and numerals 103, 103 are knuckle arms connecting the opposite ends of the rack shaft 101 to steerable front wheels 104, 104 via the tie rods 102, 102.
FIG. 10B diagrammatically illustrates a static condition of the rack shaft 101 which is achieved, for example, when the vehicle is stopped. In this condition, the rack shaft 101 is supported at two points, one corresponding in position to the position of the ball-screw mechanism 105 and the other to the position of a point of meshing engagement between the pinion 107 and the rack guide 108. The rack shaft 101 thus supported has a relatively long span (namely, the length of a part of the rack shaft 101 extending between the adjacent supporting points).
FIGS. 10C and 10D diagrammatically illustrate the behavior of the rack shaft 101 experienced when the vehicle is running. When the vehicle is running, particularly the steering wheel is turned, a reaction from road surface is transmitted through the tie rods 102, 102 to the rack shaft 101. Thus, the opposite ends of the rack shaft 101 are subjected to external forces or moments (hereinafter referred to as "moments M") acting in the same direction with the result that the rack shaft 101 is bent or flexed toward a forward or a backward direction of the vehicle, as illustrated in FIGS. 10C and 10D.
Especially, since the spring-loaded rack guide 108 is a simple spring-weight system which is constructed to support the rack shaft 101 by the force of the spring, it is not well responsive to vibrations in a higher frequency range, failing to provide a sufficient supporting force to the rack shaft 101. Accordingly, when the moments M, M from the backward direction of the vehicle act on the opposite ends of the rack shaft 101, the rack shaft 101 is bent or flexed backwardly of the vehicle in the manner shown in FIG. 10D, forcing the rack guide 108 to retract against the force of the spring.
The conventional electric power steering apparatus shown in FIGS. 10A-10D induces vibration in the rack shaft 101, the reasons for which are as follows:
(1) When each ball in the ball-screw mechanism 105 is in contact with or separated from a groove of the screw of the rack shaft 101, a collision occurs between the ball and flanks of the screw or nut whereupon a force transmitted from the ball to the screw or the nut is varied to thereby generate vibration in the rack shaft 101 (a first reason). PA1 (2) When the vehicle is running, and more particularly, when the steering wheel is turned while the vehicle is running, a reaction from road surface is transmitted to the rack shaft 101 through the tie rods 102 to thereby cause the rack shaft 101 to vibrate in the forward and backward directions of the vehicle (a second reason). PA1 (3) When the road surface reaction transmitted to the rack shaft 101 via the tie rods 102 acts to force the rack 106 in a longitudinal direction from the pinion 107 toward the ball-screw mechanism 105, the rack shaft 101 tends to bent or yield to thereby cause vibration in the rack shaft 101 (a third reason).
When a frequency of the vibration caused by the first reason equals a frequency of the vibration caused by the second or the third reason, vibration of the rack shaft 101 is amplified. Further, when the frequency of the amplified vibration approaches a natural free vibrating frequency of the rack shaft 101, the amplitude of vibration of the rack shaft 101 becomes larger due to resonant vibration.
Thus, the conventional electric power steering apparatus induces vibration in the rack shaft 101 as shown in FIGS. 10A to 10D when the ball nut mechanism 105 operates in response to a manual steering operation achieved by turning the steering wheel. Vibration of the rack 106 is amplified when the steering operation is taken for the purpose of parking the vehicle or corning of the vehicle with great acceleration where a reaction from road surface is large. The amplified vibration of rack 106 is transmitted to the passenger compartment through the steering wheel, thereby producing unpleasant noises in the passenger compartment. Vibration transmitted to the steering wheel directly provides an uncomfortable steering touch or feeling to the driver.